(1) Field of the Invention
This invention relates to processing of particulate solids as well as to an apparatus for use in this method. More specifically, the invention relates to a method for pumping or continuously compressing a particulate solid material and to a novel pump for particulate solids.
(2) Description of the Prior Art
Processing of particulate (i.e. existing in the form of particles, e.g. flakes, granules, powder etc.) solids having an apparent or "bulk" density which is lower than the structural density of the material per se, such as "plastics" scrap, frequently presents substantial processing problems when conveying and/or feeding the material into a processor, e.g. when recycling thermoplastic scrap, waste, or refuse having a low or very low bulk density, such as flakes or foamed particles, into an extruder via a chute or hopper. In fact, future economics of using and notably of recycling plastics of various kinds may be influenced significantly by the way in which low bulk density solids can be processed economically.
While various devices are known for compressing particulate solids in the form of product stream, those operating continuously tend to be complicated and costly, such as extruders, or are not suitable for general application, such as vane-type pumps.
EP-A-326,036 discloses a compacting device for feeding an extruder by means of a hollow piston that reciprocates within an elongated tubular space coaxially arranged within the extruder barrel. Optionally, a precompactor having a pair of reciprocating rams may be added but in either case compaction is effected by displacement flow, i.e. a pumping effect based upon moving element pistons or "rams" having a contact face that presses or rather compresses the particulate solid.
U.S. Pat. No. 4,102,109 discloses a wool press wherein compression of a bulky fibre mass is effected either by displacement flow caused by the vanes of a rotor or by pressure flow such as generated at the nip of a roller pair.
Finally U.S. Pat. No. 4,142,805 (Tadmor patent herein) discloses a plastics processing method using a rotary channel of the drag flow type, i.e. where a particulate material, such as a pelletized thermoplastic polymer, is compressed within an annular space or channel formed between an essentially circular inner rotor and an essentially cylindrical outer housing. The expressed aim of the approach taught in Tadmor patent is to generate frictional heat within the processed material to the extent that the latter will melt and exit from the apparatus as a liquid product.
Because the concepts underlying flow phenomena in general and drag flow in specific are explained in detail in the Tadmor patent and other publications by this author ( Z. Tadmor and I. Klein, "Engineering Principles of Plasticating Extrusion"; New York 1979; and Z. Tadmor et al, "Principles of Polymer Processing", Wiley Intersc. Publ. 1979) these publications are incorporated herein by way of reference for brevity of the specification, and the three basic types of flow that are of interest herein, i.e. drag flow, pressure flow, and displacement flow will not be discussed in detail.
Since the device disclosed in the Tadmor patent is intended mainly for processing of thermoplastic polymers, and because a molten product is to be obtained at the exit of the device, temperatures of typically in the range of 120.degree. to 250.degree. C. must be generated essentially by friction. Now, it is apparent that in order to achieve such temperatures, a maximum of friction and/or turbulence is required and this is obtained by a tortuous and locally restricted path to be followed by the material within the apparatus; local overheating may occur, notably if the properties of the processed material stream are not reasonably uniform, and this is a requirement that severely limits use of such devices in recycling of thermoplastic polymers. Further, the device disclosed in the Tadmor patent is of little use for processing materials that are thermosensitive, i.e. have a low stability against thermal degradation, such as organic substances which contain hydroxy groups and tend to split off water upon heating.
As a result, drag flow devices of the type disclosed in the Tadmor patent have been limited to processing streams of particulate thermoplastics having a reasonably high degree of uniformity, a relatively high bulk density, and a relatively low sensitivity to overheating.
Typical scrap materials, however, that could or should be processed for recovery or recycling of the polymer that constitutes the particulate material, do not normally satisfy these requirements and cannot be processed economically by prior art devices, notably if the particulate solid material has a low or very low bulk densities and/or when requiring a processing step that is to involve compression or compaction with relatively low cost machinery at reasonably low temperatures of typically below melting temperatures of thermoplastic polymer, i.e. well below 120.degree. C., preferably even below 100.degree. C.